A front-end circuit in accordance with FIG. 1 is known. The circuit includes a receiver branch, which has several partial paths that are combined into a main path on the antenna side. Each of the partial paths constitutes a reception path of various radio systems and contains a bandpass filter BP1, BP2. In the transmission path, the input to these filters can be protected by a gate circuit, which is located in the main path. The gate circuit has a transmission line TL1, a diode D2, a capacitor C1 and an inductance L1. In the transmission path, the diode D2 is closed and provides a direct current path to ground, along with the inductance L1. The series connection of the inductance, diode D2 and capacitor C1 represents a short circuit for an HF signal (transmission signal), which represents an open end at the frequency of the transmission signal on the antenna-side (ANT) of the transmission line. In the partial paths, transmission lines TL2 and TL3 are connected ahead of bandpass filters BP1, BP2, respectively. OUT1 is the output of a first and OUT2 the output of a second partial path.
It was found that the foregoing circuit does not satisfy the requirements of telephone manufacturers regarding transmission/reception isolation in the specified restricted ranges or in the receiver ICs connected in back. With insufficient TX/RX isolation, low-noise amplifiers (LNA) located in the reception paths are overloaded, particularly in the transmission path. Therefore phase errors occur in signal transmission.